Gallium-nitride-based module with enhanced electrical performance and process for making the same

ABSTRACT

The present disclosure relates to a Gallium-Nitride (GaN) based module, which includes a module substrate, a thinned switch die residing over the module substrate, a first mold compound, and a second mold compound. The thinned switch die includes an electrode region, a number of switch interconnects extending from a bottom surface of the electrode region to the module substrate, an aluminium gallium nitride (AlGaN) barrier layer over a top surface of the electrode region, a GaN buffer layer over the AlGaN barrier layer, and a lateral two-dimensional electron gas (2DEG) layer realized at a heterojunction of the AlGaN barrier layer and the GaN buffer layer. The first mold compound resides over the module substrate, surrounds the thinned switch die, and extends above a top surface of the thinned switch die to form an opening over the top surface of the thinned switch die. The second mold compound fills the opening.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 16/374,125, filed on Apr. 3, 2019, which claims the benefit of provisional patent application Ser. No. 62/652,380, filed Apr. 4, 2018, the disclosures of which are hereby incorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to a Gallium-Nitride (GaN) based module and a process for making the same, and more particularly to a GaN based module with enhanced electrical performance, and a packaging process to enhance electrical performance of GaN based modules in an economical way.

BACKGROUND

Gallium-Nitride (GaN) has long been recognized as a superior single crystal semiconductor material for use in high frequency and high power applications. The critical peak electric field in GaN semiconductors is about 12 times larger than silicon, which allows for the fabrication of very narrow gate length field-effect devices with superior channel breakdown characteristics. In addition, the electron saturation velocity is about 2.5 times larger than silicon, which allows for the engineering of field-effect transistor (FET) channel structures with very low on-resistance characteristics.

One major issue with the commercialization of GaN based FET devices has been the fact that a native low-cost GaN single crystal substrate is not feasible. Conventionally, the GaN material has been grown by epitaxy on silicon carbide (SiC) substrates due to the fact that the lattice constants of these two semiconductors are fairly similar and SiC has remarkably ideal thermal conductivity characteristics. However, the SiC substrates for GaN-on-SiC technologies are extremely expensive and are only available at 100 mm—and more recently—150 mm diameters. In addition, due to the complexities of the crystal growth of SiC, a supply chain for high volume SiC substrates does not exist currently. These drawbacks make the GaN-on-SiC technologies unsuitable for large volume commercial applications, such as those targeted for the mobile industry.

Notably, in the recent years, there has been a worldwide focus on the research of methods to grow single crystal GaN on silicon substrates, which is much more cost effective than the SiC substrates. It is, however, well known to those skilled in the art that the GaN and Si have large mismatches in the lattice constants (5.431 A for Si, 5.125 A for GaN). In addition, the silicon substrate has a number of deleterious properties for operation in radio frequency (RF) applications, such as the generation and coupling of harmonics and intermodulation distortion products.

To utilize advantages of GaN material in FET devices in an economical way, and to reduce deleterious harmonic distortion in the RF applications, it is therefore an object of the present disclosure to provide an improved and inexpensive module design with enhanced electrical performance. Further, there is also a need to keep the module size efficient.

SUMMARY

The present disclosure relates to a Gallium-Nitride (GaN) based module and a process for making the same. The disclosed GaN based module includes a module substrate, a thinned switch die, a first mold compound, and a second mold compound. The module substrate includes a substrate body having a top surface and a bottom surface. The thinned switch die is attached to the top surface of the substrate body and includes an electrode region, a number of switch interconnects, an aluminium gallium nitride (AlGaN) barrier layer, a GaN buffer layer, and a lateral two-dimensional electron gas (2DEG) layer. Each switch interconnect extends from a bottom surface of the electrode region to the top surface of the substrate body, the AlGaN barrier layer is over a top surface of the electrode region, the GaN buffer layer is over the AlGaN barrier layer, and the 2DEG layer is realized at a heterojunction of the AlGaN barrier layer and the GaN buffer layer. Herein, the first mold compound resides over the top surface of the substrate body, surrounds the thinned switch die, and extends above a top surface of the thinned switch die to form an opening over the top surface of the thinned switch die. The second mold compound fills the opening to encapsulate the thinned switch die.

According to another embodiment, the GaN based module further includes an intact controller die attached to the top surface of the substrate body. The intact controller die is configured to control operation of the thinned switch die. Herein, the intact controller die includes a device layer, a number of controller interconnects extending from a bottom surface of the device layer to the top surface of the substrate body, and a silicon controller substrate over a top surface of the device layer. In addition, the first mold compound encapsulates the intact controller die.

According to another embodiment, the GaN based module further includes a third mold compound and an intact controller die. The intact controller die is attached to the bottom surface of the substrate body and configured to control operation of the thinned switch die. Herein, the intact controller die includes a device layer, a number of controller interconnects extending from a bottom surface of the device layer to the bottom surface of the substrate body, and a silicon controller substrate over a top surface of the device layer. In addition, the third mold compound resides over the bottom surface of the substrate body and encapsulates the intact controller die.

In one embodiment of the GaN based module, the first mold compound and the third mold compound are formed from a same material.

In one embodiment of the GaN based module, the module substrate further includes connecting layers. Herein, the connecting layers are integrated in the substrate body. First portions of the connecting layers are exposed at the top surface of the substrate body, and second portions of the connecting layers are exposed at the bottom surface of the substrate body. Each switch interconnect is electrically coupled to a corresponding first portion of the connecting layers, and each controller interconnect is electrically coupled to a corresponding second portion of the connecting layers. In some applications, the GaN based module further includes a number of external contacts, each of which is electrically coupled to a corresponding second portion of the connecting layers, extends through the third mold compound, and is exposed at a bottom of the third mold compound.

In one embodiment of the GaN based module, the second mold compound has an electrical resistivity greater that 1E6 Ohm-cm.

In one embodiment of the GaN based module, the second mold compound has a thermal conductivity greater than 2 W/m·K.

In one embodiment of the GaN based module, the first mold compound and the second mold compound are formed from different materials.

In one embodiment of the GaN based module, the first mold compound is formed from a same material as the second mold compound.

In one embodiment of the GaN based module, the second mold compound is in contact with the GaN buffer layer of the thinned switch die.

In one embodiment of the GaN based module, the thinned switch die further includes a transitional layer over the GaN buffer layer and the second mold compound is in contact with the transitional layer.

In one embodiment of the GaN based module, the module substrate further includes connecting layers and a number of module contacts. Herein, the connecting layers are integrated in the substrate body and the module contacts are formed at the bottom surface of the substrate body. First portions of the connecting layers are exposed at the top surface of the substrate body, and second portions of the connecting layers are exposed at the bottom surface of the substrate body. Each switch interconnect is electrically coupled to a corresponding first portion of the connecting layers, and each module contact is electrically coupled to a corresponding second portion of the connecting layers.

According to another embodiment, the GaN based module further includes a thermally conductive film residing over at least the top surface of the thinned switch die at a bottom of the opening. Herein, the second mold compound directly resides over the thermally conductive film and fills the opening.

In one embodiment of the GaN based module, the thermally conductive film has a thermal conductivity between 5 w/m·k and 5000 w/m·k.

In one embodiment of the GaN based module, the thermally conductive film has a thickness between 0.1 μm and 100 μm.

In one embodiment of the GaN based module, the thermally conductive film is formed from one of a group consisting of chemical vapor deposition (CVD) diamond, boron nitride, aluminum nitride, alumina, and beryllium oxide.

In one embodiment of the GaN based module, the thermally conductive film has a higher thermal conductivity than the second mold compound.

According to an exemplary process, a precursor package including a number of switch dies, a package substrate, and a first mold compound is provided firstly. Herein, the package substrate includes a number of module substrates wherein an inter-module area is in between every two adjacent module substrates. Each switch die resides over a corresponding module substrate, and includes an electrode region, a number of switch interconnects extending from a bottom surface of the electrode region to the corresponding module substrate, an AlGaN barrier layer over a top surface of the electrode region, a GaN buffer layer over the AlGaN barrier layer, a lateral 2DEG layer realized at a heterojunction of the AlGaN barrier layer and the GaN buffer layer, and a silicon switch substrate over the GaN buffer layer. The first mold compound encapsulates side surfaces of each switch die, such that a top surface of the silicon switch substrate of each switch die is exposed. Next, the silicon switch substrate of each switch die is removed substantially to provide a number of thinned switch dies. There is an opening formed over each thinned switch die and a top surface of each thinned switch die is exposed at a bottom of the opening. A second mold compound is then applied to substantially fill each opening to form a GaN based package. Lastly, the GaN based package is singulated at each inter-module area to provide a number of individual GaN based modules. Herein, each GaN based module includes at least one thinned switch die and one module substrate.

In one embodiment of the process, the precursor package further includes a number of intact controller dies. Herein, each intact controller die resides over the corresponding module substrate and is configured to control operation of a corresponding switch die. Each intact controller die includes a device layer, a number of controller interconnects extending from a bottom surface of the device layer to the corresponding module substrate, and a silicon controller substrate over a top surface of the device layer. The first mold compound fully encapsulates each intact controller die, and each of the GaN based modules includes at least one thinned switch die, at least one intact controller die and one module substrate.

In one embodiment of the process, each intact controller die is shorter than each switch die.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIG. 1 shows an exemplary Gallium-Nitride (GaN) based module according to one embodiment of the present disclosure.

FIG. 2 shows an exemplary GaN based module with a thermally conductive film.

FIG. 3 shows an alternative GaN based module according to one embodiment of the present disclosure.

FIGS. 4-9 provide exemplary steps that illustrate a process to fabricate the exemplary GaN based module shown in FIG. 1.

It will be understood that for clear illustrations, FIGS. 1-9 may not be drawn to scale.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to a Gallium-Nitride (GaN) based module and a process for making the same. FIG. 1 shows an exemplary GaN based module 10 according to one embodiment of the present disclosure. For the purpose of this illustration, the exemplary GaN based module 10 includes a module substrate 12, a thinned switch die 14, a complementary metal-oxide-semiconductor (CMOS) controller die 16, a first mold compound 18, and a second mold compound 20. In different applications, the GaN based module 10 may include fewer or more dies. For instance, in some applications, the GaN based module 10 may include multiple thinned switch dies. In some applications, the GaN based module 10 may only include the thinned switch die 14 while the CMOS controller 16 is omitted. In some applications, the GaN based module 10 may further include integrated passive device dies (not shown).

In detail, the module substrate 12 may be formed from a laminate, a wafer level fan out (WLFO) carrier, a lead frame, a ceramic carrier, or the like. Herein, the module substrate 12 includes a substrate body 22, connecting layers 24, and a number of module contacts 26. The connecting layers 24 are integrated in the substrate body 22, and the module contacts 26 are formed at a bottom surface of the substrate body 22. First portions of the connecting layers 24 are exposed at a top surface of the substrate body 22 and electrically coupled to the thinned switch die 14 and the CMOS controller 16 (details are described in following paragraphs). Second portions of the connecting layers 24 are exposed at the bottom surface of the substrate body 22 and each second portion of the connecting layers 24 is electrically coupled to a corresponding module contact 26. Consequently, the connecting layers 24 may connect the thinned switch die 14 with the CMOS controller 16, and/or connect the thinned switch die 14/CMOS controller 16 to certain ones of the module contacts 26. In addition, each module contact 26 may be covered by a solder cap 28.

Both the thinned switch die 14 and the CMOS controller die 16 reside over the module substrate 12. The thinned switch die 14 includes an electrode region 30, a number of switch interconnects 32, an aluminium gallium nitride (AlGaN) barrier layer 34, a GaN buffer layer 36, and a lateral two-dimensional electron gas (2DEG) layer 38. Each switch interconnect 32 extends from the bottom surface of the electrode region 30 towards the module substrate 12, and is electrically coupled to a corresponding first portion of the connecting layers 24 at the top surface of the substrate body 22. The AlGaN barrier layer 34 is formed of AlGaN and resides over a top surface of the electrode region 30. The GaN buffer layer 36 is formed of GaN and resides over the AlGaN barrier layer 34. The lateral 2DEG layer 38 is realized at the heterojunction of the AlGaN barrier layer 34 and the GaN buffer layer 36. The lateral 2DEG layer 38 will be filled with highly mobile and abundant electrons when a suitable gate voltage in applied to a gate region of a FET transistor (more details are described below). The electrode region 30 has a thickness between 1 μm and 25 μm, each switch interconnect 32 has a height between 1 μm and 20 μm, the AlGaN barrier layer 34 has a thickness between 1 nm and 5000 nm, and the GaN buffer layer 36 has a thickness between 1 nm and 5000 nm.

Herein, the electrode region 30 may include multiple metal layers, vias, and passivation layers (not shown for simplicity) necessary for a FET process. In one embodiment, the electrode region 30 includes a gate electrode 40, a gate dielectric layer 42 in between the AlGaN barrier layer 34 and the gate electrode 40, a drain electrode 44, and a source electrode 46. The drain electrode 44 and the source electrode 46 are connected to the AlGaN barrier layer 34. Each of the gate electrode 40, the drain electrode 44, and the source electrode 46 may be electrically coupled to a corresponding switch interconnect 32 by vias (not shown). A suitable gate voltage may be applied to the gate electrode 40 through a corresponding switch interconnect 32 and vias, such that the lateral 2DEG layer 38 is filled with highly mobile and abundant electrons. In some applications, the drain electrode 44 and the source electrode 46 may extend through the AlGaN barrier layer 34 to form ohmic contacts with the lateral 2DEG layer 38, and the gate electrode 40 may be formed as a Schottky contact to the AlGaN barrier layer 34 without the gate dielectric layer 42 (not shown). When the thinned switch die 14 is “ON”, the lateral 2DEG layer 38 is conducted, and the electrons move from the drain electrode 44 to the source electrode 46. When the thinned switch die 14 is “OFF”, the lateral 2DEG layer 38 is not conducted, and there is no electron moving from the drain electrode 44 to the source electrode 46.

In one embodiment, the thinned switch die 14 may further include a transitional layer (not shown) formed over the GaN buffer layer 36. The transitional layer may be a low temperature aluminum nitride (LT-AlN) layer, a combination of multiple thin aluminum nitride and gallium nitride layers (AlN/GaN superlattices), a compositionally graded AlGaN layer, or a single low-aluminum content AlGaN layer. Consequently, in some applications, a top surface of the thinned switch die 14 may be a top surface of the GaN buffer layer 36. For other cases, the top surface of the thinned switch die 14 may be a top surface of the transitional layer (not shown). Notice that the thinned switch die 14 has no silicon-related substrate over the GaN buffer layer 36 or the transitional layer.

The CMOS controller die 16 includes a device layer 48, a number of controller interconnects 50 formed at a bottom surface of the device layer 48, and a silicon controller substrate 52 over a top surface of the device layer 48. The device layer 48 includes a CMOS controller (not shown) that is configured to control operation (“ON” and “OFF”) of the thinned switch die 14. Each controller interconnect 50 extends from the bottom surface of the device layer 48 towards the top surface of the substrate body 22, and is electrically coupled to a corresponding first portion of the connecting layers 24 exposed at the top surface of the substrate body 22. The device layer 48 has a thickness between 0.5 μm and 20 μm, and may be formed from a combination of passivation and metal layers (such as oxide, nitride, aluminum, titanium, copper, or the like). The CMOS controller die 16 is an intact die, and the silicon controller substrate 52 is an intact substrate with a thickness between 50 μm and 250 μm or between 50 μm and 750 μm.

The first mold compound 18 resides over the top surface of the substrate body 22, surrounds the thinned switch die 14, and encapsulates the CMOS controller die 16. Further, the first mold compound 18 extends beyond the top surface of the thinned switch die 14 to define an opening 54 within the first mold compound 18 and over the thinned switch die 14. Herein, the top surface of the thinned switch die 14 is exposed at a bottom of the opening 54. If the thinned switch die 14 does not include a transitional layer, the top surface of the GaN buffer layer 36 is exposed at the bottom of the opening 54. If the thinned switch die 14 includes a transitional layer, the top surface of the transitional layer (not shown) is exposed at the bottom of the opening 54. In some applications, the GaN based module 10 may further include an underfilling layer (not shown) to encapsulate the switch interconnects 32 and the controller interconnects 50. The underfilling layer resides between the top surface of the substrate body 22 and the first mold compound 18, and underfills the thinned switch die 14 and the CMOS controller die 16. The underfilling layer may be formed from the same or different material as the first mold compound 18.

The second mold compound 20 substantially fills the opening 54 to encapsulate the thinned switch die 14. The second mold compound may be in contact with the top surface of the thinned switch die 14 and may further reside over the first mold compound 18. The second mold compound 20 has a thermal conductivity greater than 2 W/m·K or greater than 10 W/m·K, and has an electrical resistivity greater than 1E6 Ohm-cm. In general, the higher the thermal conductivity of the second mold compound 20, the better the thermal performance of the thinned switch die 14. Further, the high electrical resistivity of the second mold compound 20 may improve the RF performance of the thinned switch die 14.

The second mold compound 20 may be formed of thermoplastics or thermoset materials, such as PPS (poly phenyl sulfide), overmold epoxies doped with boron nitride or alumina thermal additives, or the like. The second mold compound 20 may be formed of a same or different material as the first mold compound 18. However, unlike the second mold compound 20, the first mold compound 18 does not have thermal conductivity or electrical resistivity requirements. Herein, a portion of the second mold compound 20 may reside over a top surface of the first mold compound 18. Notice that the second mold compound 20 is separate from the CMOS controller die 16 by the first mold compound 18. A top surface of the CMOS controller die 16 is in contact with the first mold compound 18.

In some applications, the GaN based module 10 may further include a thermally conductive film 56 continuously deposited over exposed surfaces of the opening 54 and over the first mold compound 18, as illustrated in FIG. 2. Within the opening 54, the thermally conductive film 56 is immediately above the top surface of the thinned switch die 14 with no significant voids or defects. Herein, no significant voids or defects refers to no voids or defects larger than 0.1 μm between the thermally conductive film 54 and the top surface of the thinned switch die 14. In some applications, the thermally conductive film 56 only covers the top surface of the thinned flip chip die 14 (not shown). Herein, the second mold compound 20 directly resides over the thermally conductive film 56 to substantially fill the opening 54.

The thermally conductive film 56 has a high thermal conductivity between 5 w/m·k and 5000 w/m·k and a high electrical resistivity greater than 1E6 Ohm-cm. Typically, the thermally conductive film 56 has a higher thermal conductivity than the second mold compound 20. The thermally conductive film 56 may be formed of chemical vapor deposition (CVD) diamond, aluminum nitride, boron nitride, alumina, beryllium oxide, or the like. Depending on different deposition stresses, different deposited materials, and different applications of the thinned switch die 14, the thermally conductive film 56 has different thicknesses varying from 0.1 μm to 100 μm. For a CVD diamond material, which has an extremely high conductivity greater than 2000 w/m·k, a 1 μm or greater thickness of the thermally conductive film 56 is extremely effective for the heat dissipation management of the thinned switch die 14. For a boron nitride material, which has a high conductivity between 50 w/m·k-100 w/m·k, a 5 μm-10 μm thickness of the thermally conductive film 56 is desirable.

To reduce the footprint of a module, the thinned switch die 14 and the CMOS controller die 16 may be placed at opposite sides of a module substrate. FIG. 3 shows an alternative GaN based module 10A, which includes an alternative module substrate 12A, the thinned switch die 14, the CMOS controller die 16, the first mold compound 18, the second mold compound 20, and a third mold compound 58. The alternative module substrate 12A may be formed from a laminate, a wafer-level-fan-out (WLFO) carrier, a lead frame, a ceramic carrier, or the like. In one embodiment, the alternative module substrate 12A includes the substrate body 22 and the connecting layers 24 without the module contacts 26. The connecting layers 24 are integrated in the substrate body 22 and configured to connect the thinned switch die 14 with the CMOS controller die 16. First portions of the connecting layers 24 are exposed at the top surface of the substrate body 22 and electrically coupled to the thinned switch die 14. Second portions of the connecting layers 24 are exposed at the bottom surface of the substrate body 22 and electrically coupled to the CMOS controller die 16 (details are described in following paragraphs).

The thinned switch die 14 is attached to the top surface of the substrate body 22, while the CMOS controller die 16 is attached to the bottom surface of the substrate body 22. Each switch interconnect 32 of the thinned switch die 14 is electrically coupled to a corresponding first portion of the connecting layers 24 at the top surface of the substrate body 22. Each controller interconnect 50 of the CMOS controller die 16 is electrically coupled to a corresponding second portion of the connecting layers 24 at the bottom surface of the substrate body 22.

In this embodiment, the first mold compound 18 resides over the top surface of the substrate body 22 and surrounds the thinned switch die 14. The first mold compound 18 extends beyond the top surface of the thinned switch die 14 to define the opening 54 within the first mold compound 20 and over the thinned switch die 14. Herein, the top surface of the thinned switch die 14 is exposed at the bottom of the opening 54. The second mold compound 20 substantially fills the opening 54 to encapsulate the thinned switch die 14. Herein, the second mold compound 20 may be in contact with the top surface of the thinned switch die 14, and may further reside over the first mold compound 18. The third mold compound 58 resides over the bottom surface of the substrate body 22 and encapsulates the CMOS controller die 16. In addition, the alternative GaN based module 10A may also include external contacts 60, each of which is electrically coupled to a corresponding second portion of the connecting layers 24 at the bottom surface of the substrate body 22. Each external contact 60 extends through the third mold compound 58 and is exposed at bottom of the third mold compound 58. Each external contact 60 is separate from the CMOS controller die 16 by the third mold compound 58.

FIGS. 4-9 provide exemplary steps to fabricate the exemplary GaN based module 10 shown in FIG. 1. Although the exemplary steps are illustrated in a series, the exemplary steps are not necessarily order dependent. Some steps may be done in a different order than that presented. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated in FIGS. 4-9.

Initially, a number of device groups 62 are attached to a package substrate 12F as depicted in FIG. 4. The device groups 62 may be attached to the package substrate 12F via an adhesive material (not shown). The package substrate 12F includes a number of the module substrates 12, and a number of inter-module areas 64. Each inter-module area 64 is in between every two adjacent module substrates 12 and has a small width. Herein, each device group 62 is attached to a corresponding module substrate 12, and does not reside on any inter-module area 64. As described above, each module substrate 12 includes the substrate body 22, the connecting layers 24, the module contacts 26, and the solder caps 28.

In this embodiment, each device group 62 includes a switch die 14F and the CMOS controller die 16. In different applications, there might be fewer or more devices included in one device group 62. The switch die 14F includes the electrode region 30, the switch interconnects 32 formed at the bottom surface of the electrode region 30, the AlGaN barrier layer 34 over the top surface of the electrode region 30, the GaN buffer layer 36 over the AlGaN barrier layer 34, the lateral 2DEG layer 38 realized at the heterojunction of the AlGaN barrier layer 34 and the GaN buffer layer 36, and a silicon switch substrate 66 over the GaN buffer layer 36. As such, the backside of the silicon switch substrate 66 is a top surface of the switch die 14F. In some applications, the switch die 14F may further include a transitional layer (not shown) formed between the silicon switch substrate 66 and the GaN buffer layer 36. The transitional layer may be a low temperature aluminum nitride (LT-AlN) layer, a combination of multiple thin aluminum nitride and gallium nitride layers (AlN/GaN superlattices), a compositionally graded AlGaN layer, or a single low-aluminum content AlGaN layer. In addition, each switch interconnect 32 of the switch die 14F extends from the bottom surface of the electrode region 30 towards its corresponding module substrate 12, and is electrically coupled to a corresponding connecting layer 24.

Herein, the switch die 14F has a thickness between 20 μm and 500 μm, and the silicon switch substrate 66 has a thickness between 20 μm and 500 μm. The CMOS controller die 16 has a thickness between 20 μm and 500 μm, and the silicon controller substrate 52 has a thickness between 20 μm and 500 μm. Note that the CMOS controller die 16 is always shorter than the switch die 14F.

Next, the first mold compound 18 is applied to the package substrate 12F, covers the top surface of each substrate body 22, and encapsulates each switch die 14F and each CMOS controller die 16, as illustrated in FIG. 5. The first mold compound 18 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, or screen print encapsulation. The first mold compound 18 may be an organic epoxy resin system or the like, which can be used as an etchant barrier to protect the switch die 14F and the CMOS controller die 16 against etching chemistries such as potassium hydroxide (KOH), sodium hydroxide (NaOH), and acetylcholine (ACH). A curing process (not shown) is then used to harden the first mold compound 18. The curing temperature is between 100° C. and 320° C. depending on which material is used as the first mold compound 18.

The first mold compound 18 is then thinned down to expose the silicon switch substrate 66 of each switch die 14F, as illustrated in FIG. 6. The thinning procedure may be done with a mechanical grinding process. Since the CMOS controller die 16 has a lower height than the switch die 14F, the silicon controller substrate 52 of the CMOS controller die 16 is not exposed and is still encapsulated by the first mold compound 18.

The following step is to substantially remove the silicon switch substrate 66 as shown in FIG. 7. The removal of the silicon switch substrate 66 from the switch die 14F provides the thinned switch die 14 and forms the opening 54 that is within the first mold compound 18 and over the thinned switch die 14. Herein, if the switch die 14F includes a transitional layer between the silicon switch substrate 66 and the GaN buffer layer 36, this transitional layer may be partially or completely removed. In the thinned switch die 14, there is no portion of the silicon switch substrate 66 left over the GaN buffer layer 36, and the GaN buffer layer 36 (or the transitional layer) is exposed at the bottom of the opening 54. Removing substantially the silicon switch substrate 66 may be provided by an etching process with a wet/dry etchant chemistry, which may be TMAH, KOH, ACH, NaOH, or the like.

The second mold compound 20 is then applied to substantially fill each opening 54 to form a GaN based package 68, as illustrated in FIG. 8. The second mold compound 20 may be applied by various procedures, such as sheet molding, overmolding, compression molding, transfer molding, dam fill encapsulation, and screen print encapsulation. The second mold compound 20 may directly reside over the top surface of the thinned switch die 14. If there is no transitional layer in the thinned switch die 14, the second mold compound 20 may directly reside over the GaN buffer layer 36. In some cases, the second mold compound 20 may further reside over the first mold compound 18. A curing process (not shown) is followed to harden the second mold compound 20. The curing temperature is between 100° C. and 320° C. depending on which material is used as the second mold compound 20. A top surface of the second mold compound 20 may be planarized by a mechanical grinding process (not shown).

Herein, the GaN based package 68 includes a number of the GaN based modules 10, which share the package substrate 12F, the first mold compound 18, and the second mold compound 20. Lastly, the GaN based package 68 is singulated at each inter-module area 64 to provide individual GaN based modules 10, as illustrated in FIG. 9. Each individual GaN based module 10 at least includes the thinned switch die 14, the CMOS controller die 16 and the module substrate 12.

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A method comprising: providing a precursor package having a plurality of switch dies, a package substrate, and a first mold compound, wherein: the package substrate includes a plurality of module substrates, wherein an inter-module area is in between two adjacent module substrates of the plurality of module substrates; each of the plurality of switch dies resides over a corresponding module substrate, and comprises an electrode region, a plurality of switch interconnects extending from a bottom surface of the electrode region to the corresponding module substrate, an aluminium gallium nitride (AlGaN) barrier layer over a top surface of the electrode region, a gallium nitride (GaN) buffer layer over the AlGaN barrier layer, a lateral two-dimensional electron gas (2DEG) layer realized at a heterojunction of the AlGaN barrier layer and the GaN buffer layer, and a silicon switch substrate over the GaN buffer layer; and the first mold compound encapsulates side surfaces of each of the plurality of switch dies, wherein a top surface of the silicon switch substrate of each of the plurality of switch dies is exposed; removing substantially the silicon switch substrate of each of the plurality of switch dies to provide a plurality of thinned switch dies and form an opening over each of the plurality of thinned switch dies, wherein a top surface of each of the plurality of thinned switch dies is exposed at a bottom of the opening; applying a second mold compound to substantially fill each opening to form a GaN based package; and singulating the GaN based package at each inter-module area to provide a plurality of GaN based modules, wherein each of the plurality of GaN based modules includes one module substrate and at least one thinned switch die.
 2. The method of claim 1 wherein the precursor package further comprises a plurality of controller dies, wherein: each of the plurality of controller dies resides over the corresponding module substrate and is configured to control operations of a corresponding switch die; each of the plurality of controller dies comprises a device layer, a plurality of controller interconnects extending from a bottom surface of the device layer to the corresponding module substrate, and a silicon controller substrate over a top surface of the device layer; and the first mold compound fully encapsulates each of the plurality of controller dies.
 3. The method of claim 2 wherein: each of the plurality of controller dies is shorter than each of the plurality of switch dies; and each of the plurality of GaN based modules includes one module substrate, at least one thinned switch die, and at least one controller die.
 4. The method of claim 1 wherein the second mold compound has an electrical resistivity greater that 1E6 Ohm-cm.
 5. The method of claim 1 wherein the second mold compound has a thermal conductivity greater than 2 W/m·K.
 6. The method of claim 1 wherein the first mold compound and the second mold compound are formed from different materials.
 7. The method of claim 1 wherein the first mold compound is formed from a same material as the second mold compound.
 8. The method of claim 1 wherein the second mold compound is in contact with the GaN buffer layer of each of the plurality of thinned switch dies.
 9. The method of claim 2 wherein providing the precursor package comprises: attaching the plurality of switch dies and the plurality of controller dies to the package substrate, wherein each of the plurality of switch dies and each of the plurality of controller dies reside on a corresponding module substrate of the package substrate; applying the first mold compound over the package substrate to encapsulate each of the plurality of switch dies and each of the plurality of controller dies; and thinning down the first mold compound to expose the silicon switch substrate of each of the plurality of switch dies, wherein each of the plurality of controller dies is intact.
 10. The method of claim 9 wherein the first mold compound is thinned down by a mechanical grinding process.
 11. The method of claim 1 wherein the silicon switch substrate of each of the plurality of switch dies is removed by an etching process with an etchant chemistry, which is at least one of a group consisting of TMAH, KOH, NaOH, ACH, and XeF₂. 